FIG. 1 shows a prior-art communication system 100 which is part of a larger communications network and is used for establishing data transfer connections to base stations 101, 102, and 103 of a mobile telephone network. The communication system includes data communication devices 104, 105, and 106 which may be e.g. Internet Protocol (IP) routers, Multiprotocol Label Switching (MPLS) devices and/or Ethernet switches. Each data communication device may consist of a single apparatus or a combination of a plurality of interconnected apparatuses. The data communication devices are interconnected through a regional communications network 107. The regional communications network 107 may include other data communication devices and communication links between them. The communication system includes a gateway device 108 providing a data transfer connection to a communication backbone network 109, represented e.g. by the global Internet. The communication system includes a mobility management entity (MME) 110 arranged to maintain up-to-date information about the location of a radio terminal 111 in the radio communication network comprised of base stations 101 to 103. The mobility management entity 110 is connected to the gateway device 108 either directly or through a communications network. The mobility management entity 110 may also be embedded in the gateway device 108. The radio terminal 111 can be a mobile phone or palmtop computer, for example.
The mobility management entity 110 is arranged to produce, based on the location of the radio terminal 111, control data on the basis of which the data communication device 104 is able to establish a logical data transmission tunnel 112 between the data communication device 104 and base station 101 in response to a situation in which the radio terminal is under the base station 101. Similarly, data communication device 105 is able to establish a logical data transmission tunnel 113 between the data communication device 105 and base station 102 in response to a situation in which the radio terminal is under the base station 102, and data communication device 106 is able to establish a logical data transmission tunnel 114 between the data communication device 106 and base station 103 in response to a situation in which the radio terminal is under the base station 103. The producing of said control data is often called a communications network mobility function. Said logical data transmission tunnels may be e.g. GTP-U (General Packet Radio Service Tunneling Protocol—User plane messages) tunnels, GRE (Generic Routing Encapsulation) tunnels, IPinIP (Internet Protocol in Internet Protocol) tunnels or data transmission tunnels implemented using Ethernet frames.
In the situation depicted in FIG. 1 we may assume, for instance, that the radio terminal 111 is first under base station 101 and there exists an ongoing communication session between the radio terminal and a communications network, which session can comprise e.g. the downloading of a web page from a data source in the communications network to the radio terminal in accordance with the Transmission Control Protocol (TCP). Said communication session may be originally established between the radio terminal 111 and a data source 115 behind the communication system 100. The data source 115 can be a server for maintaining certain web pages, for example. In said communication session, the payload data transmitted to the radio terminal, e.g. a web page, may also be stored in data sources 116 and 117 in connection with the data communication devices 104 and 105 each of which may include a cache memory device, for example. In such a situation, the payload data is transmitted to the radio terminal 111 advantageously from data source 116 instead of data source 115 to produce a load as small as possible upon the communications network. It is of course possible that data source 116 is a server and said communication session is originally established between the radio terminal 111 and the data source 116. In a situation in which the radio terminal 111 is handed over from base station 101 to base station 102 the logical data transmission tunnel 112 is removed and a logical data transmission tunnel 113 is established based on the control data produced by the mobility management entity 110. The communication session is either transferred from data source 116 to data source 117 or said communication session is disconnected and a new one is established between the radio terminal 111 and data source 117. To transfer the communication session between data sources 116 and 117 or to continue the communication session with a new communication session there is typically needed in the radio terminal 111 a dedicated client agent which can be software-based, for example. Document US20060268780, for instance, discloses a technical solution according to the prior art where a client agent is required in the radio terminal for controlling a communication session as said radio terminal is handed over from one base station to another.